Technologies associated with power adapters for devices such as laptop computers and cellphones are fast advancing as new protocols and devices are unveiled. In addition to wide voltage range and high efficiency, small size is a valued feature for power adapters. With new semiconductor materials and advances in fabrication, size of semiconductors has been reduced significantly. Passive components, which may occupy more than 80% of the converter volume, however, generally cannot be reduced in size. With the recent development of USB Power Delivery, power adapters are designed for 15-60 W. At such power level, power factor correction is not mandatory. Thus, a full bridge rectifier D1-D4 and a buffering electrolytic capacitor Cin as shown in FIG. 1 are often used to convert the 100-240 VAC input into DC voltage. In some compact designs, the bulk electrolytic capacitor may take as much as a quarter of the total converter volume. Thus, reducing the capacitor value and size can have significant influence on the converter size.
A prior approach to reduce the energy storage of Cin was proposed in Published European Patent Application No. EP2750274A1. Compared to a conventional full bridge approach as shown in FIG. 1, that approach required an additional capacitor and two additional diodes, and more complex control logic, which increased cost and size of the circuit, and effectively cancelled much of the reduction in overall power converter size that was achieved by reducing the size of the input capacitor.